How do you make submarines? - Luke, 5, Western Washington

The next time you’re in the bathtub, turn a cup upside down on the water. Push down on it as hard as you can. See if you can get it to sink below the water.

It’ll be difficult to do! The air inside the cup makes it lighter than the water. But what happens if you turn the cup on its side, allowing water to rush in? You’ll see it’s easier to push underwater.

Those same basic forces make a submarine work.

That’s what I learned from Ian Richardson, an engineer at Washington State University. He is very curious about how liquids and solids interact. He has even helped NASA work on a submarine to someday go to Titan, one of Saturn’s moons.

Buoyancy describes an object’s ability to float. It’s key to making a submarine. “It’s pretty easy to get something to sink and easy to get something to float,” Richardson said. “To get something to stay in the middle of a liquid is very challenging.”

Ships float because they’re full of air. Air is lighter than the water around them. But submarines dive and rise. They’re able to do this because they control their weight using a combination of water and air.

Ballast tanks are the secret. These special containers sit inside the submarine and control its buoyancy.

“These tanks either let water in or they blow water out with air, and that’s how they control their buoyancy. They dive or surface based on how much water is in their ballast system,” Richardson said. When air enters, the submarine gets lighter and rises. But when the tanks fill with water, the submarine becomes heavier and sinks.

There are other important parts of a submarine’s design. Special parts create oxygen for passengers to breathe. The inside temperature stays steady to protect sensitive technology inside. And they’re usually made of strong metal, like steel or titanium.

Maybe someday you’ll help design these important features. Until then, you can make your own miniature submarine. All you need is an empty plastic bottle, 4 heavy coins, a flexible straw, and tape.

First, have a grown-up help you make holes in the bottle: three on its side, and one in its cap. Screw the cap on. These holes will allow water and air into your submarine.

Next, tape the coins next to the row of holes in the side. Two should go near the top of the bottle, and two near the bottom. They’ll make the submarine heavier, but keep it balanced.

Now, take your flexible straw and put it in the hole on the bottle’s cap. Make sure the straw is pointed up, so it will stick out of the water.

When you’re ready to test your submarine, set it in water. As water enters, you’ll see the submarine sink. But if you blow into the straw, air gets pushed inside. The submarine rises.